The ways in which excitable cells alter their function over long periods (days, weeks, or months) are largely unknown. Experiments at nerve-muscle junctions have shown that long-term interactions between pre - and postsynaptic cells may be mediated by as yet unspecified 'trophic factors'. Recent evidence, however, has shown that the past history of activity of denervated or innervated muscle fibers also influences their properties: activity can affect the area of the cell which is responsive to transmitter agent, and its ability to initiate spontaneous action potentials. These findings raise the question of whether the properties of neurons are similarly affected by activity or the lack of it. The purpose of this work will be to study this question in mammalian neurons (sympathetic ganglion cells) using the techniques of electrophysiology, and light and electron microscopy. Specifically, I will try to establish what changes ganglion cells undergo following interuption of the preganglionic fibers, and whether these changes can be controlled by experimentally changing the level of activity of the neurons. In related studies I will examine the effect of activity on reinnervation of ganglion cells, and long-term changes in neurons following axotomy. The significance of this work lies in the possibility of providing some basis for understanding how neurons in the mammalian central nervous system alter their properties for prolonged periods.